A shell-and-tube heat exchanger is an indirect heat exchanger. Heat is transferred between a fluid passing through the tubes of a tube bundle (the tube side) extending in the heat exchanger shell, and a fluid passing through the space outside the tubes (the shell side). Details of the shell-and-tube heat exchangers can for example be found in Perry's Chemical Engineers' Handbook, 6th edition, 1984, McGraw-Hill Inc., page 11-3 to 11-21.
A particular type of heat-exchanger known as two-shell-pass heat exchanger has been developed for improved transfer of heat in a given shell size. In this type of heat exchanger a generally cylindrical outer tube is provided internally with an axially and longitudinally extending partition baffle. Such shell types include the two-pass shell with longitudinal baffle, the split-flow shell, and the double split-flow shell in Perry's. The longitudinal baffle subdivides the interior of the shell into two separate longitudinally extending compartments that normally communicate at one end of the shell, so that the fluid flow in the shell passes twice along the length of the shell.
For most efficient heat exchange the baffle should form a relatively tight seal along both of its longitudinal rims so that flow between the compartments is only possible in the intended regions, that is at the end or ends of the shell.
Typically, such a structure has been formed by using a rectangular partition plate having a width slightly smaller than the internal diameter of the wall of the shell so that the longitudinal outer rims of this plate are spaced slightly radially inwardly from the inner wall surface of the shell, when the plate is positioned on a diametral plane.
Several types of longitudinal seals have been developed in the past. Except for sufficient sealing, it is also desired that a longitudinal seal allows easy mounting in a heat exchanger shell, and is cost-efficient. A good compromise has for example been found in the baffle seal profiles developed and marketed under the name T4 by Kempchen & Co. GmbH of Oberhausen, Germany. Principles of these seals are also described in USA patent specification No. 4215745, which also discusses other prior art seals.
The known longitudinal seals comprise a U-shaped flange that faces inwardly into the heat exchanger and that is sized to snugly receive the longitudinal baffle. A sealing member at the opposite side of the seal comprises an outwardly extending pair of flanges that elastically presses against the inner wall of the shell.
In many cases a two-shell-pass heat exchanger is not an optimal arrangement. For example, when an existing single pass heat exchanger is to be retrofitted with new internals, the positions of the fluid inlet and outlets of the shell are located at opposite ends longitudinally along the heat exchanger shell, and that can normally not be changed. For a two-pass arrangement, however, shell inlet and outlet should be arranged at the same longitudinal end of the shell.
A three-shell-pass arrangement, in which two longitudinal baffles are arranged so that the fluid flow in the shell meanders three times back and forth the length of the shell, would solve this problem. However, there is considerable hesitation against installation of such a layout, because the design will only realize its high heat-exchange capacity if the longitudinal seals are reliable enough to prevent fluid leakage between passes in the shell side. Although the Kempchen seals are good, they cannot guarantee that leakage is prevented.